Threat of hung parliament looms, PM warns

The threat of a hung parliament looms over the federal government, with Prime Minister Scott Morrison warning Wentworth by-election voters of economic uncertainty if they boot the Liberals.

Mr Morrison says a Liberal loss in the Sydney seat could mean a hung parliament that would create "unnecessary uncertainty in our economy and the stability of our government".

He admitted the Sydney seat, which was formerly held by Malcolm Turnbull, was a three-horse race between his own party, Labor and independent Kerryn Phelps.

"If you don't vote for the Liberal candidate at this election ... you risk a hung parliament," Mr Morrison told reporters in South Australia on Saturday.

While singing the praises of Liberal candidate Dave Sharma, the prime minister urged Liberals who were considering voting for Dr Phelps to "think about that carefully".

"(Dr Phelps) is clearly running strongly second and if she continues in that place, she can still be a long way behind on primaries and then she can come over the top and win that by-election," he said.

Mr Sharma agreed with the prime minister: "It's simple maths - if anyone other than the Liberal candidate is elected then we will have a hung Parliament, meaning greater uncertainty for our economy and our country."

But federal Opposition Leader Bill Shorten said the October 20 by-election was an opportunity for fed up voters to send a message to the government.

"If you are not happy with the way this government has been running things for the last two-and-a-half, and indeed five years, send them a message - 'not this time'," he told reporters in Sydney on Saturday.

Dr Phelps also hit out at the "revolving door of leadership" caused by the major parties in government and criticised both Labor's and the coalition's climate change policies.

"The Morrison government is hopelessly divided on climate science and what to do about it, while Labor's plan doesn't go far enough," she told AAP in a statement on Saturday.

Former Prime Minister Malcolm Turnbull resigned from his seat after losing the Liberal leadership at the end of August, sparking the by-election.